1. Field of the Invention
The present invention relates to an autofocus lens device applied to an electronic still camera having a solid-state image pickup element having a large number of pixels.
2. Description of the Related Art
A focusing lens of an autofocus type is driven by a motor in accordance with an AF signal from a camera and moved forward or backward in a direction along a photographing optical axis to perform a focusing operation.
In such a driving mechanism for the autofocusing by using a lead screw device, for example, a screw axis is connected to an output axis of the motor, the screw axis is screwed in a nut member mounted on a focusing lens holding frame while the focusing lens holding frame is movably joined to a guide bar provided in parallel with the photographing optical axis for stopping the rotation of the focusing lens holding frame. Accordingly, when the motor drives, the focusing lens holding frame moves forward or backward in the direction along the photographing optical axis by forwarding action of the screw axis and straight guiding action of the guide bar, thus performing the focusing operation.
FIG. 5 is an enlarged view of a main part, which shows a mounting structure for a nut member 2 or a focusing lens holding frame 1. In FIG. 5, the nut member 2 is formed into an inverted U-shape, and on inner surfaces thereof facing each other, screwing portions 4, 4 in which a screw axis 3 is screwed are threaded. The nut member 2 thus formed is inserted in the nut mounting portion 5 formed on the focusing lens holding frame 1 with an opening portion downward. Thereby the nut member 2 is mounted on the focusing lens holding frame 1.
However, since the above-described conventional lens device is configured so that, as shown in FIG. 5, gaps 6, 6 are formed between the nut mounting portion 5 of the focusing lens holding frame 1 and the nut member 2 in view of production errors of both. Accordingly, there is a problem that the nut member 2 is inclined or misaligned against the nut mounting portion 5.
In recent electronic still cameras, higher precision in the focusing operation is required as the number of pixels of a CCD, which is a solid-state image pickup element, increases. However, if the nut member 2 is inclined or misaligned as described above, forwarding precision is declined; consequently, there is a drawback that it is not possible to perform focusing operation with high precision.
The present invention has been developed in view of the above-described circumstances, and an object of the present invention is to provide a lens device capable of performing focusing operation with high precision.
In order to achieve the above-described object, the present invention is directed to a lens device, comprising: a lens holding frame; a screw axis provided in parallel with an optical axis, the screw axis moving the lens holding frame forward and backward along the optical axis; a nut member formed into an inverted U-shape, in which the screw axis is screwed; and a nut mounting portion formed on the lens holding frame, on which the nut member is mounted, the nut mounting portion transferring a moving force of the nut member in the direction along the optical axis to the lens holding frame, the moving force being generated by rotation of the screw axis, wherein an engaging portion for positioning the nut member against the nut mounting portion is formed on the nut mounting portion formed on the lens holding frame.
According to the present invention, the nut member is engaged with the engaging portion formed on the nut mounting portion of the lens holding frame to position the nut member against the nut mounting portion, thus preventing inclination, misalignment or deformation of the nut member against the nut mounting portion. Consequently, the forwarding precision of the lead screw device is improved; therefore, the focusing operation can be performed with high precision.